Actually heres an even easier way to get your DN. From any computer with Microsoft AD powershell cmdlets installed run the following command

get-aduser {admin username} | select-object distinguishedname

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padraic CarronIT SecurityAuthor Commented: 2015-02-13

Thanks that seemed to work, but when I go to save it fails on the Alternate domain name.
I have put in the paramaters:
Domain name = mydomain.ie
Alternate Domain = mydomain
The error is "This domain is in use by the mydomain.ie"